(1) Field of the Invention
The present invention relates to a small glass electrode and a process for the preparation thereof. More particularly, the present invention relates to a small glass electrode formed by utilizing a micro-machining technique and a process for the preparation thereof.
(2) Description of the Related Art
A glass electrode is easily usable as a sensor for determining the hydrogen ion (H.sup.+) concentration in an aqueous solution, and is widely and generally used.
The determination of the H.sup.+ concentration is required not only in ordinary chemical experiments but also in fermentation control and in the medical field.
Furthermore, a biosensor fabricated by combining a glass electrode with either enzymes or microorganisms can be used for determining various chemical compounds.
For example, glucose reacts with dissolved oxygen with the aid of a catalyst called glucose oxidase and is oxidized to gluconolactone. Changes of the H.sup.+ concentration during this reaction are measured, and the glucose concentration can be determined from the changed quantities. According to a similar principle, the urea concentration can be determined.
In the glass electrode, the H.sup.+ concentration is measured by utilizing the electroconductivity of glass. Namely, the phenomenon that when a glass film having a thickness of about 100 .mu.m and an electrical resistance of several hundred M.OMEGA. is placed in a solution, a voltage difference is produced according to the pH value of the solution which is utilized.
FIG. 1 illustrates the structure of a sensing portion of a conventional glass electrode, which comprises a reference electrode 2 composed of silver or silver/silver chloride (Ag/AgCl), an internal solution 4 such as a potassium chloride (KCl) solution having a certain concentration, and a spherical sensing glass film 6 formed on the top end.
When this glass electrode is immersed in a solution containing H.sup.+, in response to the active quantity (ai) of H.sup.+, a potential is generated according to the Nernst equation: EQU E=const+(RT/F)lnai (1)
wherein E represents the potential of the glass electrode, R represents the gas constant, T represents the absolute temperature, and F is the Faraday constant.
Accordingly, the H.sup.+ concentration is determined by the above equation.
However, the commercially available glass electrode has a size similar to that of a fountain pen, as shown in FIG. 1, and the glass electrode is formed by glazing and it is expensive.
An ion-sensitive electric field effect transistor (abbreviated to "ISFET") has been developed as a small H.sup.+ concentration sensor. Since a photolithographic technique of semiconductor production is utilized for its formation, the size thereof can be reduced.
In a device immersed in an aqueous solution, such as ISFET, insulation of the substrate is important.
Accordingly, many elements formed on a silicon (Si) substrate are diced into chips and a silicon nitride (Si.sub.3 N.sub.4) film is formed on the peripheries of the chips to effect insulation, or an SOS (silicon-on-sapphire) substrate is used. Alternatively, there is adopted a method in which a thin film transistor (TFT) is formed on a glass substrate. However, the increased price of such a structure cannot be avoided and therefore, the sensor cannot be manufactured at a low price.
The conventional glass electrode formed by glazing is large size and expensive. In an ISFET formed by the photolithography of an Si substrate, insulation is indispensable and a price increase is inevitably caused by the necessity of maintaining the insulation, and reduction of the price is not easy.
Under this background, development of another method of providing a practical glass electrode of small size and low price is desired.